Gameplay input refers to the game taking some kind of data from an exterior source. Gameplay inputs are the essences of what allow us to play games. Understanding our options when designing games can be all it takes to create more unique experiences.
The most common form is a boolean input. A button press is a boolean type data...an on/off type of data, if you will. If playing with one button, the only way to enter data to a game is by pressing and depressing it. You can still make a lot of variation by using patterns, rhythm, by having the player hold or tap for differing lengths of time. By adding multiple buttons, you allow them to do button combinations, or hold down different buttons simultaneously. By arranging the buttons in a different configuration, such as the up-down-left-right buttons, they take on a meaning to the player.
Another type of input is the analog input. Joysticks are analogs, but so are the velocities of the different axes of a wii remote for example. The position of the mouse cursor on either axis might be considered analog. Analog refers to a control that always sends the game a number within a range, rather than simply being on or off.
Some interesting things to note about inputs is that the way they are collected can completely change the experience. Such as taking data from the accelerometers in a Wii remote versus detecting how far a joystick has been pushed. The game machine takes this data in as numbers in either case, but the difference can be billions of dollars of sales. The important thing is, creating unique experiences with satisfying controls that come intuitively to the player.
There are many other types of inputs, and I wonder how many others we can discover. Consider for example, database input...Facebook games draw input from user profiles. This alters the game, linking content to your profile. I have seen some games that analyze wavelengths of mp3's to generate custom content. World of Warcraft uses a real-time clock to determine what time it should be in the game.
It's amazing to me to see how many ways buttons and joysticks have been used. When you think about it, we take for granted what you can do with buttons. (How many of you knew for example that the PS2 controllers' buttons are actually analog...they can sense how hard they are being pressed, not just boolean information). What types of inputs are underused? How can we arrange or change existing inputs to create new types of games? There are so many possibilities, and games have barely scratched the surface!